# Projectile motion — Thinking about forces on a curve ball

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• Aurelius120
In summary, the conversation discussed the dynamics of a ball thrown in a curved trajectory in the horizontal plane and the forces involved in creating this motion. The Magnus effect and air resistance were mentioned as factors that affect the ball's path. The conversation then shifted to discussing the rotation of a knife when thrown in the air and how it is caused by an initial force but does not require a force to maintain it. The concept of torque and angular velocity were also mentioned in relation to this phenomenon.
Aurelius120
TL;DR Summary
It is about a horizontal projectile exhibiting curvillinear motion
When a ball is thrown such that it moves in a curved trajectory in the horizontal plane, it amuses me to think of its dynamics.

In motion of a ball thrown upwards the force of gravity gives it a parabolic trajectory

However when the ball is thrown to curve and hit a target, (in the horizontal plane) it cannot be gravity that causes it to curve because it acts downwards

The force exerted by us also ceases once the ball leaves our hand and moves ahead

Then what is it that gives the required centripetal force to exhibit curvillinear motion

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Gravity is not the only force acting on the ball. There is also air to consider.

Aurelius120 said:
Summary:: It is about a horizontal projectile exhibiting curvillinear motion

Then what is it that gives the required centripetal force to exhibit curvillinear motion
To move in a horizontal path, the ball must be spinning with some vertical axis component. The differential air speed of the left and the right hand sides of the ball are then different. That moves the point of stagnation at the front of the ball to one side, so the ball does not experience a balanced airflow. That circulation of air causes the "sideways" force on the ball.
https://en.wikipedia.org/wiki/Magnus_effect

You mean this can never happen on the Moon?

docnet
Aurelius120 said:
You mean this can never happen on the Moon?
i've heard the moon also has a negligible amount of atmosphere, so I wouldn't say never!

Aurelius120 said:
You mean this can never happen on the Moon?
Compare the atmospheric pressure on Earth to that on the Moon. (You won't see anyone throwing curve balls on lunar missions.)

Is this also responsible for a knife rotating after it is thrown in air?

Aurelius120 said:
Is this also responsible for a knife rotating after it is thrown in air?
Not sure what you're thinking of. While air resistance does affect the motion, the knife rotates because it was given a spin before leaving the hand. And it generally doesn't follow a curved path in the horizontal plane.

The knife can rotate if you spin it. Nevertheless, if you take away the air, and just concentrate on the center of mass of the knife, you will observe tha knife center of mass will follow a parabolic path If you could hurl the knife far enough from the Earth, though, the path would be an ellipse.

But if I spin it
I only give it an initial force
Which ceases as soon as the knife leaves my hand
So there is no force to further rotate
Then why should it rotate?
Could it be due to velocity of approach?

Aurelius120 said:
But if I spin it
I only give it an initial force
Which ceases as soon as the knife leaves my hand
So there is no force to further rotate
Then why should it rotate?
Could it be due to velocity of approach?
A force (actually a torque about the center of mass) is needed to start the knife rotating, but once it's rotating it will keep on rotating without needing that force. A torque is needed to change the rotation, not to maintain it.

Compare that to force and velocity: A net force is needed to change an object's velocity, but not to maintain it.

It does not have to do with velocity. The spinning has to do with angular velocity. When you spin it, (before it leaves your hand, you are rotating it), the angular velocity, think of it as a rotation rate, continues to rotate the knife, even after it leaves your hand.
This is like when you stop applying force, that does not mean the speed is zero. The speed is maintained even without the force. In the same way the rotation is maintained even though there is no force (acutally to be precise, we are talking about torque here not force) on the knife, after it leaves your hand.

## 1. What is projectile motion?

Projectile motion is the motion of an object through the air, under the influence of gravity and any other external forces. This type of motion is typically seen in objects that are thrown, kicked, or launched into the air.

## 2. How does a curve ball move in the air?

A curve ball moves in the air due to the forces of gravity and air resistance acting on it. The spin of the ball creates a difference in air pressure on either side of the ball, causing it to curve in the direction of the lower air pressure.

## 3. What forces act on a curve ball?

The main forces that act on a curve ball are gravity, air resistance, and the Magnus effect. The Magnus effect is the force created by the spin of the ball, which causes it to curve in the air.

## 4. How does the speed of a curve ball affect its trajectory?

The speed of a curve ball affects its trajectory by determining how much time it spends in the air. The faster the ball is thrown, the less time it has to curve before reaching the batter. This means that a faster curve ball will have a smaller curve than a slower one.

## 5. Can a curve ball defy the laws of physics?

No, a curve ball cannot defy the laws of physics. While it may appear to do so due to its curved path, it is simply following the laws of motion and the forces acting on it. The spin and speed of the ball, along with external forces, determine its trajectory and movement in the air.

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